This invention relates to an automatic volume adjusting apparatus which can correctly adjust the volume of sound from a loudspeaker in accordance with the variation of ambient noise level.
Many attempts have been made to automatically adjust the volume of sound from the audio output device of a television set, a radio, a tape recorder, a loudspeaker and the like, in accordance with the variation of ambient noise level. And many methods have been proposed to achieve such an automatic volume adjusting. Notable among them are as follows:
(a) A microphone is used to detect a composite sound constituted by the ambient noise and the sound from a loudspeaker. Only the ambient noise component of the composite sound is removed and detected from a composite sound signal generated by a sound signal circuit. The ambient noise component is used to control the gain of an audio output device. PA1 (b) The ambient noise is detected when a loudspeaker does not generate sound. According to the level of the ambient noise one of static circuits such as switches which have a self-holding function is selected thereby to control the volume of sound generated by the loudspeaker.
With the method (a), however, it is difficult to remove and detect only the ambient noise component from the composite sound constituted by the ambient noise and the sound from the loudspeaker, that is, to completely cancel out the electric signal from the sound signal circuit and the sound generated by the loudspeaker. This is because the sound from the loudspeaker involves in phase delay, the loudspeaker and microphone differ in frequency characteristic, or for some other reasons. If the electric signal and the sound could be cancelled out almost completely, the ambient noise would be masked by the sound from the loudspeaker when the sound volume increases to a certain level. As a result, the microphone would fail to detect the ambient sound, and the sound volume could no longer increase. Consequently, the volume adjustment would become unstable.
In the method (b) the volume adjustment is carried out somewhat statically. The method (b) is not therefore practical in case the ambient noise changes abruptly or periodically while a loudspeaker is generating sound. With the method (b) it is next to impossible to adjust the sound volume particularly when the loudspeaker generates a continuous sound such as music. This is because in this case the loudspeaker is rarely silent so that the volume of sound from it is maintained at constant level.
Accordingly, it is an object of this invention to provide an automatic volume adjusting apparatus which is so designed as to increase or reduce the volume of sound generated by a loudspeaker accurately in accordance with the level of the ambient noise, regardless of masking phenomenon which occurs when the volume of sound from the loudspeaker increases to a certain level according to the ambient noise level.